The inherent optical properties of a water body (mesoscale), namely, the absorption coefficient, the scattering coefficient, and the volume scattering function combine with the radiant distribution above the sea to yield the apparent optical properties (Preisendorfer, 1961). The radiative transfer equation is the link between these two classes of optical properties. Locally, the inherent properties of seawater are governed by, and strictly result from, the sum of the contributions of the various components, namely, the water itself, the various particles in suspension able to scatter and absorb the radiant energy, and finally the dissolved absorbing compounds. Analyzing these contributions is an important goal of optical oceanography. Among these particles, the phytoplanktonic cells, with their photosynthetic pigments, are of prime importance, in particular in oceanic waters far from terrestrial influence. They also are at the origin of other kinds of particles, such as their own debris, as well as other living “particles” grazing on them (bacteria, flagellates and other heterotrophs). Studying optics at the level of single cells and particles is therefore a requirement for a better understanding of bulk optical properties of oceanic waters. Independently of this goal, the study of the individual cell optics per se is fundamental when analyzing the pathways of radiant energy, in particular the light harvesting capabilities and the photosynthetic performances of various algae or their fluorescence responses. The following presentation is a guidline for readers who will find detailed studies in the classic books Light Scattering by Small Particles by van de Hulst (1957) and Light and Photosynthesis in Aquatic Ecosystems by Kirk (1983), as well as in a paper dealing specifically with the optics of phytoplankton by Morel and Bricaud (1986). This chapter is organized according to the title, with first a summary of the relevant theories to be applied when studying the interaction of an electromagnetic field with a particle, and then, as a transition between this scale and that of in vitro experiments, some results concerning the optical behavior of pure algal suspensions; finally the more complicated situations encountered in natural environments are briefly described to introduce the “nonlinear biological” effect (Smith and Baker, 1978a) upon the optical coefficients for oceanic waters, and to examine some of the empirical relationships, as presently available, between the pigment concentration and the optical properties of the upper ocean at mesoscale and global scale.
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